1. Field of the Invention
This invention relates to an electrolytic cell and in particular to an electrolytic cell which is provided with liquor recirculating means.
Electrolytes, for example, aqueous solutions of alkali metal chlorides, particularly sodium chloride, are electrolysed on a vast scale throughout the world in order to produce products such as chlorine and aqueous alkali metal hydroxide solution. The electrolysis may be effected in an electrolytic cell comprising a plurality of anodes and cathodes with each anode being separated from the adjacent cathode by a separator which divides the electrolytic cell into a plurality of anode and cathode compartments.
2. Description of the Prior Art
The electrolytic cell may be of the diaphragm or membrane type. In the diaphragm type cell the separators positioned between adjacent anodes and cathodes are microporous and in use aqueous electrolyte passes through the diaphragms from the anode compartments to the cathode compartments of the cell. In the membrane type cell the separators are essentially hydraulically impermeable and in use ionic species are transported across the membranes between the anode compartments and the cathode compartments of the cell.
For example, where aqueous alkali metal chloride solution is electrolysed in an electrolytic cell of the diaphragm type the solution is charged to the anode compartments of the cell, chlorine which is produced in the electrolysis is removed from the anode compartments of the cell, the alkali metal chloride solution passes through the diaphragms and hydrogen and alkali metal hydroxide produced by electrolysis are removed from the cathode compartments, the alkali metal hydroxide being removed in the form of an aqueous solution of alkali metal chloride and alkali metal hydroxide. Where an aqueous alkali metal chloride solution is electrolysed in an electrolytic cell of the membrane type the solution is charged to the anode compartments of the cell and chlorine produced in the electrolysis and depleted alkali metal chloride solution are removed from the anode compartments, alkali metal ions are transported across the membranes to the cathode compartments of the cell to which water or dilute alkali metal hydroxide solution may be charged, and hydrogen and alkali metal hydroxide solution produced by the reaction of alkali metal ions with water are removed from the cathode compartments of the cell.
The electrolysis may be effected in an electrolytic cell of the filter press type which may comprise a large number of alternating anodes and cathodes, for example, fifty anodes alternating with fifty cathodes, although the cell may comprises even more anodes and cathodes, for example up to one hundred and fifty alternating anodes and cathodes.
The electrolytic cell may be provided with an inlet header through which electrolyte, for example aqueous alkali metal chloride solution, may be charged to the anode compartments of the cell, and with an outlet header through which products of electrolysis may be removed therefrom. Also, the electrolytic cell may be provided with an outlet header through which products of electrolysis may be removed from the cathode compartments of the cell, and optionally, e.g. in the case of a membrane type cell, with an inlet header through which liquor, for example water or other fluid, may be charged thereto.
Electrolytic cells may be fitted with means for recirculating the liquors to the anode and/or cathode compartments of the cell. For example, in an electrolytic cell of the membrane type in which aqueous alkali metal chloride solution is electrolysed the solution is charged to the anode compartments of the cell through an inlet header and chlorine and depleted aqueous alkali metal chloride solution are removed therefrom through an outlet header. The electrolytic cell may be equipped with means for recirculating the depleted alkali metal chloride solution, or a part thereof, back to the anode compartments of the cell for re-use therein. Prior to effecting the recirculation the gaseous chlorine may be separated from the defleted alkali metal chloride solution, and the depleted solution may be mixed with alkali metal chloride or with fresh more concentrated aqueous alkali metal chloride solution prior to recirculation of the solution to the anode compartments. Recirculation of the aqueous alkali metal chloride solution enables the solution to be re-used and it ensures that a high conversion of alkali metal chloride may be effected without the conversion in a single pass through the anode compartments being so high that unacceptable concentration gradients result in the solution within the anode compartments of the cell, and between the solutions in different anode compartments of the cell, with consequent loss in current efficiency. Furthermore, as the solution removed from the cell is at high temperature the fresh solution may be at relatively low temperature. Indeed, it may be unnecessary to heat the fresh solution.
In such an electrolytic cell in which aqueous alkali metal chloride solution is electrolysed water is charged to the cathode compartments of the cell through an inlet header and hydrogen and dilute aqueous alkali metal hydroxide solution are removed therefrom through an outlet header. The electrolytic cell may be equipped with means for recirculating the alkali metal hydroxide solution, or a part thereof, back to the cathode compartments of the cell in order to increase the concentration of alkali metal hydroxide in the solution. Prior to effecting the recirculation the gaseous hydrogen may be separated from the alkali metal hydroxide solution, and the solution may be mixed with water prior to recirculation. If such recirculation was not effected it would be difficult to produce a solution of alkali metal hydroxide of high concentration, and if such a solution of high concentation was produced without recirculation there would be substantial concentration gradients in the solution within the cathode compartments of the cell, and between the solutions in different cathode compartments of the cell, with a consequent unacceptable loss in current efficiency.
The recirculation may be effected by means of suitable pipework positioned externally of the electrolytic cell. For example, the outlet header from the anode compartments of the cell may be connected to a branched outlet pipe and part of the depleted electrolyte removed from the anode compartments of the electrolytic cell may be passed through the branched pipe to an inlet pipe, which is in turn connected to the inlet header of the anode compartments of the cell, and through which fresh electrolyte may also be charged to the anode compartments of the cell. Part of the electrolyte removed from the anode compartments of the electrolytic cell may be removed from the cell through the branched pipe. Similarly, the outlet header from the cathode compartments of the cell may be connected to a branched outlet pipe and part of the liquor removed from the cathode compartments of the electrolytic cell may be passed through the branched pipe to an inlet pipe, which is in turn connected to the inlet header of the cathode compartments of the electrolytic cell, and through which liquor, such as water, may also be charged to the cathode compartments of the cell. Part of the liquor removed from the cathode compartments of the electrolytic cell may be removed from the cell through the branched pipe.
An electrolytic cell having pipework positioned externally of the cell and through which liquors may be recirculated is described in U.S. Pat. No. 3,856,651. The recirculation system relies for its effectiveness on the gas-lift effect, and in the patent there is described a bipolar cell having a tank positioned on top of the cell to which chlorine-containing aqueous sodium chloride solution is passed from the anode compartments of the cell. Chlorine is separated from the solution in the tank, and the solution is removed from the tank and mixed with fresh, more concentrated sodium chloride solution and returned to the anode compartments of the cell via an externally positioned pipe.
The recirculation may also be effected within the anode compartments of an electrolytic cell, or within the cathode compartments of an electrolytic cell, by means of downcomers positioned in the compartments of the cell, for example, by means of a downcomer positioned between a pair of electrode plates in an electrode compartment of a cell. Such recirculation also relies for its effectiveness on the gas-lift effect.
An electrolytic cell in which there is internal liquor recirculation is described in U.S. Pat. No. 4,557,816. In the patent there is described a duct which facilitates downward flow of electrolyte and which is positioned in a space to the rear of an electrode, the duct comprising a horizontal portion having a lower opening near the inlet for fresh electrolyte and a vertical portion in communication with the horizontal portion and having an upper opening near the outlet for the depleted electrolyte.